1. Field of the Invention
The present invention relates to wireless power feeding and, more particularly, to overcurrent protection therefor.
2. Description of Related Art
A wireless power feeding technique of feeding power without a power cord is now attracting attention. The current wireless power feeding technique is roughly divided into three: (A) type utilizing electromagnetic induction (for short range); (B) type utilizing radio wave (for long range); and (C) type utilizing resonance phenomenon of magnetic field (for intermediate range).
The type (A) utilizing electromagnetic induction has generally been employed in familiar home appliances such as an electric shaver; however, it can be effective only in a short range. The type (B) utilizing radio wave is available in a long range; however, it has small electric power. The type (C) utilizing resonance phenomenon is a comparatively new technique and is of particular interest because of its high power transmission efficiency even in an intermediate range of about several meters. For example, a plan is being studied in which a receiving coil is buried in a lower portion of an EV (Electric Vehicle) so as to feed power from a feeding coil in the ground in a non-contact manner. Hereinafter, the type (C) is referred to as “magnetic field resonance type”.
The magnetic field resonance type is based on a theory published by Massachusetts Institute of Technology in 2006 (refer to U.S. Patent Application Publication No. 2008/0278264). In U.S. Patent Application Publication No. 2008/0278264, four coils are prepared. The four coils are referred to as “exciting coil”, “feeding coil”, “receiving coil”, and “loading coil” in the order starting from the feeding side. The exciting coil and feeding coil closely face each other for electromagnetic coupling. Similarly, the receiving coil and loading coil closely face each other for electromagnetic coupling. The distance (intermediate distance) between the feeding coil and receiving coil is larger than the distance between the exciting coil and feeding coil and distance between the receiving coil and loading coil. This system aims to feed power from the feeding coil to receiving coil.
When AC power is fed to the exciting coil, current also flows in the feeding coil according to the principle of electromagnetic induction. When the feeding coil generates a magnetic field to cause the feeding coil and receiving coil to magnetically resonate, high current flows in the receiving coil. At this time, current also flows in the loading coil according to the principle of electromagnetic induction, and power is taken from a load connected in series to the loading coil. By utilizing the magnetic field resonance phenomenon, high power transmission efficiency can be achieved even if the feeding coil and receiving coil are largely spaced from each other (refer to U.S. Patent Application Publication No. 2009/0072629).
In the wireless power feeding, high current (overcurrent) may flow in a power feeding side when a distance between the feeding coil and receiving coil is large. In a non-contact power feeder disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 11-164497, power is fed to a pickup coil in a non-contact manner.
Temperature of the pickup coil is monitored by a temperature sensor, and an abnormal signal is output when a level of an output of the temperature sensor exceeds a certain value. The temperature of the pickup coil increases when the high current flows in the pickup coil. That is, the overcurrent can be detected by the increase in the temperature of the pickup coil. In a case of occurrence of the overcurrent, the pickup coil is short-circuited to stop power supply.
In an invention disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 11-164497, the temperature sensor needs to be mounted to the coil. Further, this invention provides a method that monitors not a current value itself, but an increase in temperature associated with an increase in current, so that responsiveness in the overcurrent detection is low. This may cause a high load on an electronic circuit during a time period from when the overcurrent starts flowing until a protection function is activated.
The present invention has been made in view of the above problem, and a main object thereof is to achieve protection of an electronic circuit against the overcurrent in wireless power feeding with a simple configuration.